In recent years, since scaling of semiconductor devices has been advanced, there has been proposed a nonvolatile semiconductor memory device of a three-dimensional structure type that includes memory cells forming a multilayer structure. According to the nonvolatile semiconductor memory device of the three-dimensional structure type, films to serve as memory elements are stacked in the vertical direction and are processed together, so that a number of memory cells are formed within a unit plane. In this case, the cost per one bit can be reduced.
When the nonvolatile semiconductor memory device of the three-dimensional structure type is manufactured, a stacked body is formed such that a sacrificial film and a spacer film, each of which is made of an insulating material, are alternately stacked each in a plurality of layers, then memory holes are formed through the stacked body by etching, then a channel and so forth are formed in each memory hole, and then each sacrificial film is replaced with a conductive material for forming a gate structure in the nonvolatile semiconductor memory device.
As the number of stacked layers in the stacked body is increased and the thickness of the layers is increased, it comes to take a longer time to form the memory holes. Particularly, as a position in the stacked body is closer to the lower side, the amount of etchant to be supplied thereto is reduced due to the presence of reaction products, and the etching rate is thereby lowered. Further, along with an increase in the etching time, bowing ends up being caused.